This invention relates to the field of deep-draft semi-submersible offshore platforms for the drilling of oil wells and natural gas wells and the production of oil and or gas from such wells. Specifically, the present invention relates to a type of deep-draft semi-submersible platform known as an extendable draft platform, or “EDP.” More particularly, the present invention relates to mechanisms and arrangements for the connection of a plurality of buoyancy columns to a floating deck of the platform.
The development of deep water offshore oil and gas fields, such as are found in the Gulf of Mexico and the North Sea, present substantial challenges to the industry. Early production schedule requirements favor inshore integration and commissioning and a year-round deployment capability. Moreover, the ability to use so-called “dry trees” and steel catenary risers (“SCRs”) requires that the motion of the deployed structures be relatively small, even in rough seas.
In response to these challenges, a number of different types of tethered, floating platforms have been proposed for use in offshore oil and gas exploration and production, such as are described in the following patent publications: U.S. Pat. No. 6,024,040; U.S. Pat. No. 6,196,767; and U.S. Pat. No. 6,524,032. The teachings of these publications are incorporated herein by reference.
One type of offshore platform that has met with commercial success in deep water applications is the semi-submersible platform. Conventional semi-submersible platforms, however, are subject to motions that make it difficult, or even impossible, to support the various types of risers that are employed in such platforms. Deep draft semi-submersible platforms have been proposed that would exhibit superior motion characteristics. One type of deep draft semi-submersible platform is known as the extendable draft platform, or “EDP.” The typical EDP comprises a buoyant equipment deck having a plurality of openings (“leg wells”) through the deck. The deck may conveniently be rectangular or triangular, with a leg well at each corner or apex, although other configurations may be used. Installed in each of the leg wells is a buoyancy column that can be ballasted (e.g., with seawater). The columns are initially installed in a raised position, and then lowered to a submerged position when the EDP has been moved to a deep water site. Each column is divided by transverse internal bulkheads and horizontal flats (decks) into a plurality of compartments, the compartments including means for introducing water into them for ballasting purposes when the columns are lowered to their submerged positions. Attached to the bottom of the columns is a heave plate pontoon assembly that helps to stabilize the EDP against the heave action of waves and swells. Examples of prior art EDPs are disclosed in U.S. Pat. No. 6,718,901—Abbott et al. and U.S. Pat. No. 6,024,040—Thomas, the disclosures of which are incorporated herein by reference.
Once the columns are lowered to their submerged position, the tops of the columns must be securely fixed to the deck. It is important that the attachment or connection of the columns to the deck be secure enough to withstand the strong forces that current and waves often apply to the EDP. The attachment or connection mechanism must also allow the downward movement of the column relative to the deck as the column is moved to its submerged position prior to the attachment or connection of the column to the deck. In other words, the attachment or connection mechanism must not interfere with the column lowering process. While prior art approaches (such as rack-and-pinion lowering systems which can be locked in various ways) have provided satisfactory results, further improvements in the connection/attachment mechanisms have been sought, so as better to achieve the intended results, particularly when a rack-and-pinion lowering system is not employed.